Feature: Detection of the Java Class File version

[jorsol]

Sometimes it might be useful to have a way to read the class file version of a compiled class to know which version of Java was used to compile that class.

One example of such use case is in the maven-compiler-plugin, where it should be possible to detect the class file version of a previous compilation and compare it with the current version of the target/release, this way it allows to detect if the class needs recompilation or not.

[rfscholte]

You have my interest. Not sure, but this might be related to the classfile-api .

[jorsol]

I was thinking of something a lot simpler than using ASM or similar, will try to provide a PR soon.

[rfscholte]

It looks like with the classfile api it will be as easy as

ClassModel classModel = ClassModel.of(bytes);
int majorVersion = classModel.majorVersion();
int minorVersion = classModel.minorVersion();

There's no target release yet, so it makes sense to add something similar. With a multirelease jar we can also redirect to the official class-api.

sandbox sourcerepo: https://github.com/openjdk/jdk-sandbox/tree/classfile-api-branch